I've done some research lately, especially on the drama genre. I don't see why anyone would bother to write original drama scripts when at least 80-95% of drama movies are based on novels, and the small percentage that aren't based on novels consists of only comedy and musicals. I decided to check out other genres as well, and guess what? I see the same pattern, especially currently. So many Hollywood movies and even indie films are based on novels , so what's the point in writing a screenplay? Might as well be a novelist.

Many writers of screenplays are creative people with a story they
want to tell. They aren't writing based on percentages. Have you
researched how many screenplays are written and how many get
made? I don't see why anyone would bother writing a screenplay.
Have you done research on how many novels are written and how
many are published? Are the percentages better?

I didn't do the deep research you did but I looked at the top ten
drama films of 2016 by box office, popularity (IMdB) and critics
(Rotten Tomatoes) and found that 4 in 10 were adaptations. That's
not 80-95%, that's less than 50%.

I'd love to know more about your research. Is this 80-95% based
on all of the 2000's? Did you include only studio films? How did you
define "drama"? You might be mathematically correct. However you
don't seem to understand the creative mind of a writer.